1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. Besides, an oxide semiconductor has been attracted attention.
For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) and having an electron carrier concentration of lower than 1018/cm3 is disclosed (see Patent Document 1).
A transistor including an oxide semiconductor is known to have a problem of low reliability because of high possibility of change in electric characteristics, although the transistor including an oxide semiconductor can be operated at higher speed than a transistor including amorphous silicon and can be manufactured more easily than a transistor including polycrystalline silicon. For example, there are shifts of the threshold voltage in the transistor before and after a BT test performed under light. On the other hand, Patent Documents 2 and 3 each disclose a technique of preventing charge trapping at the interface of an oxide semiconductor layer by providing an interfacial stability layer which is in contact with a channel formation region of the oxide semiconductor layer, in order to suppress the shift of the threshold voltage of the transistor including an oxide semiconductor.